The present invention relates to a method for fabricating a semiconductor device and more particularly to a method for forming a semiconductor device having recess channels.
As the degree of integration in an integrated circuit semiconductor device increases and design rules rapidly decrease, the difficulty in assuring the stable operation of transistors increases. For example, transistor gate widths and channel lengths are reduced rapidly. These causes short channel effects and can generate a punch through between the source and drain of the transistor. Punch through is one of the common reasons for the device malfunction. In order to overcome the short channel effect, various methods for assuring a sufficient channel length without increasing design rules have been developed. Particularly, a method for fabricating a semiconductor device having bulb-type recess channels using a two-stage etching process to increase the length of the channels in comparison with the line width of the gates has been proposed.
FIGS. 1 and 2 are sectional views illustrating a conventional method for fabricating a semiconductor device having recess channels.
Referring to FIG. 1, bulb-type trenches 104 for recess channels are first formed in a semiconductor substrate 100 in which active regions are defined by an isolation structure 102. In order to provide the cell transistors with suitable threshold voltages, channel ion implantation is performed on the entire semiconductor substrate 100 surface. Thereby forming the impurity regions 106 surrounding the bulb-type trenches 104 for recess channels.
Referring to FIG. 2, gate stacks 118 are then formed on the bulb-type trenches 104 for recess channels, and source/drain regions 120 are formed by injecting impurities into the semiconductor substrate 100. The gate stack 118 is made up of a gate insulating film pattern 110, a conductive film pattern 112, a metal film pattern 114, and a hard mask film pattern 116.
Region A shows where the impurity region 106 overlaps the source/drain region 120. The overlapping regions cause a localized increase in the concentration of impurities, which leads to an increase in the electric fields. Thereby, the threshold voltage is changed and the refresh characteristic of a semiconductor device is degraded.